From the prior art it is known to provide a secondary injection in such a way that the injected fuel still ignites and as a result of the combustion in the expansion phase the exhaust gas temperature rises. This means, in accordance with the teaching of EP 0 621 400 B1, that a catalytic converter disposed in the exhaust gas line is heated from a cold state up to an operating temperature more quickly.
From DE 100 61 796 A1, by purposeful use of secondary injection a response characteristic of a downstream exhaust-gas turbocharger is improved in that the subsequently injected fuel is converted substantially only in the region of the catalytic converter and brings about a marked rise of temperature and enthalpy of the exhaust gas there. The exhaust-gas turbocharger is consequently run up faster, which particularly during acceleration is important. During acceleration, and hence above all given previous travel at less than normal speed, the turbocharger at the exhaust side initially does not have the correct quantity of exhaust gas to generate the charge-air pressure required for the acceleration operation. This slow response characteristic is also known colloquially as turbo lag. By transposing the teaching from DE 100 61 796 A1 this turbo lag is reduced. Also known, moreover, are methods that by additionally injecting fuel into the late working stroke or into the expulsion cycle of an internal combustion engine supply substantially unburnt fuel to the catalytic converter disposed in the exhaust gas line. In this method, by measuring the temperature characteristic the exothermal reactions arising in the catalytic converter are determined. The determination of the exothermal reactions is in turn evaluated as a measure of the conversion capability and hence the operating ability of the catalytic converter. These methods work with excess air and may therefore be used in all lean-burn engines and in particular in gasoline direct injection engines and diesel engines. For this purpose, however, an additional exhaust-gas temperature sensor is required.
A method of determining a catalytic converter temperature on the basis of various operating variables of the drive system using a physical model is known from DE 102 54 477 B3. In this specification it is however also pointed out that the provision of a concrete temperature sensor for measuring the catalytic converter temperature works much more accurately.
The previously mentioned teachings for diagnosing the effectiveness of catalytic converters have the fundamental drawback that they provide results only after a long time delay. This is due i.a. to the basic approaches selected, which are based on a balancing of the states in the control mode and in a separately triggered test mode given suitable operating parameters.
A further drawback of known methods is that in arrangements having catalytic converters disposed very close to the engine, for example in the respective cylinder head, in an exhaust manifold or immediately upstream of a turbocharger, the extra equipment outlay and/or processing outlay of these methods is many times higher. This is true especially in the case of modern approaches to engine design, where there is a separate catalytic converter disposed in each outlet channel, these being known as cylinder head catalytic converters.